In international relations, governance arrangements are often characterized as marked by varying degrees of formality, with classic international institutions such as the Security Council or the IMF lying at the formal end of the spectrum and myriad other arrangements at the informal end. In turn, the proliferation of informal governance mechanisms is glossed as an ad hoc response to new problems, or to the practical difficulties of negotiating new multilateral arrangements. However, supposedly informal governance arrangements can be highly patterned and, arguably, are the consequence of predictable nondecisions in the classic Bachrach-Baratz sense. This paper develops a formal model of the emergence and persistence of informal governance relationships, and illustrates it with two examples: one about narcotics and the other about cybersecurity.